Title: The New Instrument for Solar Diameter Measurements at Rio de Janeiro
Author: Reis Neto, Eugenio
Affil: Observatorio Nacional, Brasil
Email: eugenio@on.br
Authors: Andrei, Alexandre H.; Delmas, Christian (*); Bourget, Pierre J.A.; Penna, Jucira L.; Morand, Frederic(*); Laclare, Francis(*) and Jilinski, Evgueni G.;
Affils: (*) CERGA - Observatoire de la Cote d'Azur
Abstract: Secular and annual variations of the solar diameter, on a scale of tenths of arc-second, have been detected by several studies. Smaller scale rapid variations also show up in the monitoring series. In this case, however, they do not stand clearly above the noise from observational errors and instrumental limitation. Precision of the order of the milli arc-second, in time scales of few days, are required to better investigate the rapid diameter variations, and their relationship with the solar constant, solar events and climate response.
A major step is going to be taken by the launching of the PICARD mission. On ground, the CERGA group will continue the observations with DORAYSOL, as well as to make pre-mission observations with the PICARD-SOL instrument and to take complementary atmosphere measurements with MISOLFA. A key characteristic of DORAYSOL is that the measurements are absolute, in metrological sense, with the main spatial reference independent of the optics.
At, Observatorio Nacional, Rio de Janeiro, the series of daily measurements will be kept with a new instrument in development. This will have Newtonian optics, with 3m focal length and 10cm effective aperture. The prism is designed to take advantage of the major axis of the entrance pupils to obtain an increasing resolution. No spring is used in the prism, the stability being attained by a balancing scheme. A CCD camera will be mounted on the focus of the only mirror, what minimizes the optical defects. Light transportation within the instrument will be along a vertical path, thus preventing turbulence through a stratified regime of air couch. The observations would be made either on direct mode or switching images by a shutter. A set of three filters will enable observations in different wavelengths and a focusing device will rend the results compatible, by compensating for the changing optics and atmosphere lensing.